This invention relates to aircraft control systems and more particularly to a system for use during landings which presents to the pilot a command bar indication on a head-up display.
During visual landing approaches, when outside cues are degraded, it is difficult for the pilot to maintain a desired fixed angle path to touchdown. Particularly during black hole approaches, and with no horizon reference, an excessive deviation from the path can occur before the pilot can detect it visually. To solve this, a horizontal bar projected onto a transport lens in the pilot's line-of-sight was developed to provide a reference indicating his position relative to the desired approach path. This is called a head-up display(HUD). Pilots found it difficult to avoid vertical oscillation when flying with the aid of only this position reference. Therefore, most HUD designs include aircraft direction of motion (velocity vector) as well as position.
The following discloses a new design that does not require velocity vector computation to aid pilots in damping vertical oscillation. The damping results from a signal, derived electronically from measured aircraft motion parameters. The signal deflects the bar relative to the desired fixed angle according to an optimized control law. It provides anticipation of aircraft motion. It does not have the weaknesses of prior designs which either are subject to wind error (constant wind or windshear), or require an inertial navigation system(INS). The total bar drive signal of the disclosed design consists of a signal representative of a gyro stabilized fixed angle summed with lead compensation signals. Various lead compensation circuits are included to improve dynamic characteristics. Aircraft altitude rate is coupled through a high pass filter (washout circuit) to remove the steady state descent rate and produce a rate compensation signal. This is adequate, but improved dynamic response is gained with the combination of aircraft pitch angle coupled through a high pass filter, pitch rate, and normal acceleration. Also, to match the effect of increased angular sensitivity to vertical deviations from the fixed path as the runway is approached, the gain of the compensation terms are increased as a function of reduced altitude. Normal acceleration can be substituted for differentiated altitude rate in the high pass filter to avoid the dynamic problems that can result from poor altitude rate measurement caused by the effect of changing angle of attack on the static port. Satisfactory compensation can also be derived using normal acceleration only, which is coupled into a lag-lead network. This can be used in aircraft that approach to land while holding constant pitch, such as short take off and landing(STOL) aircraft approaching on the backside of the drag curve. Other combinations of lead compensation signals can also produce good dynamic characteristics.
Prior designs of the landing approach HUD depend upon two basic functions, distance the aircraft is off the desired approach path (indicated by position error .eta..sub.e) and direction of motion of the aircraft (velocity vector .gamma.). Several variations of this design have been developed. The simplest HUD is an aimsight depressed a fixed angle below the horizontal (.eta..sub.c FIG. 1). The aimsight is a horizontal bar projected onto a transparent viewing lens in the pilot's line-of-sight. The bar appears superimposed on the outside world. It lines up with the runway aimpoint only when the aircraft is on the selected fixed angle path that terminates at the aimpoint. Whether or not the aircraft is on the path, the aimpoint direction is defined by an angle .eta..sub.a (FIG. 1). The distance the aircraft is off the path is reflected by the angular displacement of the bar from the aimpoint (.eta..sub.e FIG. 1). .eta..sub.e = .eta..sub.c - .eta..sub.a. The pilot acquires and stays on the path by maneuvering the aircraft to hold the bar on his aimpoint, that is, he tries to keep .eta..sub.e equal to zero. Vertical oscillation can occur while tracking this position reference when visual cues are limited, such as at night where only approach and runway lights are available for visual orientation.
To solve this the HUD was designed to include, in addition to the position reference, a direction of motion reference, or velocity vector angle (.gamma.). Velocity vector is computed either from angle of attack (.alpha.), or from vertical speed (h) divided by horizontal speed. Horizontal speed is derived either from indicated airspeed (IAS) or ground speed (gnd spd). See FIG. 2. In various HUD designs .gamma. is combined with .eta..sub.c in different ways: One design uses two bars, one showing the depression angle of the desired path (.eta..sub.c), and the other, the depression angle of the velocity vector (.gamma.). Another design electronically combines in a single bar a fraction of .eta..sub.c with a fraction of .gamma., the total of these two fractions equaling one, i.e., Bar depression angle = K.gamma. + (1-K) .eta..sub.c.
Each of the possible variations, indicated above, contains some disadvantage.
.alpha. and IAS are airmass measurements and as such are subject to wind error. That is, there is a difference between true glidepath angle relative to the ground (.gamma..sub.t) and glidepath angle relative to the airmass (.gamma..sub.w). As shown in FIG. 2 a headwind will cause .gamma..sub.w to be a shallower angle than the true descent angle (.gamma..sub.t). By holding .gamma..sub.w on his aimpoint, the pilot continuously directs the aircraft on an approach angle that is steeper than the direct line to the aimpoint. This results in the pursuit curve shown in FIG. 2.
An additional disadvantage of using .alpha. to compute .gamma. results from the short term effect of vertical windshear. A quick increase in downdraft (or reduction in updraft) causes a decrease in .alpha. until aircraft inertia is overcome by the downdraft and aircraft descent rate increases accordingly. The reduced .alpha. causes .gamma..sub.w to be less negative, moving the bar up which directs the pilot to fly down to hold the bar on the aimpoint. It is apparent that this is the wrong response to this windshear.
Where horizontal speed is the ground speed computed in an inertial navigation system (INS), the velocity vector provides accurate control. However, two weaknesses remain; (1) the aircraft must have an INS and (2) the combination of .eta..sub.c and .gamma. still does not produce accurate optimized control.